Mainstream internal combustion engines are engines fuelled by one fuel, typically a liquid fuel like diesel or gasoline. However, liquid fuelled internal combustion engines generate a considerable share of pollutants released into the environment, such as oxides of nitrogen (NOx) and particulate matter (PM). Such emissions can be reduced by substituting some of the diesel or gasoline with cleaner-burning gaseous fuels such as natural gas, pure methane, ethane, liquefied petroleum gas, lighter flammable hydrocarbon derivatives, hydrogen, and blends of such fuels. Gaseous fuels are generally defined herein as fuels that are gaseous at atmospheric pressure and zero degrees Celsius.
An advantage of some of these gaseous fuels is that, as a resource, they are more widely distributed around the world and with respect to natural gas, the amount of proven reserves is much greater, compared to proven oil reserves. Methane can also be collected from renewable sources such as vent gases from garbage dumps, sewage treatment plants and agricultural operations. Hydrogen can be produced with electricity generated from renewable sources such as wind mills and hydro-electric dams.
However the fuelling infrastructure for gaseous fuels is still under development. Therefore it can be advantageous for automotive manufacturers to offer engines that have the flexibility to be fuelled with either a conventional liquid fuel or a less expensive and cleaner burning gaseous fuel. In this disclosure an engine with this fuel flexibility, for example that can be fuelled with either gasoline or natural gas is defined as a “bi-fuel” engine. This is to distinguish bi-fuel engines from dual fuel engines which are defined herein to mean engines that can be fuelled with two different fuels at the same time.
Possible arrangements for bi-fuel engines include injecting either natural gas or liquid fuel directly in the combustion chamber of the engine or in the engine's air intake port. The fuels have different fluid properties, including for example, gaseous fuels being compressible fluids versus liquid fuels being virtually incompressible, and significant differences and variability in mass densities resulting from the compressibility of gaseous fuels. While fuel injection valves for liquid fuels and gaseous fuels exist today, a fuel injection valve designed for liquid fuel is not suitable for injecting the same amount of energy when fuelling an engine with a gaseous fuel. Conventional bi-fuel engines normally use separate fuel-specific fuel injectors, but this requires finding space to mount two different fuel injection valves and can be a problem, for example if it is desired to inject both fuels directly into the combustion chamber. Using a single injection valve for alternatively injecting a gaseous fuel or a liquid fuel is an advantageous solution when one considers the space limitations of conventional engines.
While gaseous fuels and liquid fuels have generally the same energy density (the amount of energy per unit mass), they have very different mass densities (the mass per unit volume), with the mass density of gaseous fuels being much lower than that of liquid fuels. The mass flow rate of a fluid depends on the mass density p, the available flow area A, and the fluid velocity or discharge speed V according to the formula:{dot over (m)}=ρVA
Furthermore, since gaseous fuels are compressible fluids, their mass densities decrease linearly with decreasing pressure. Liquid fuels such as gasoline and diesel fuel are virtually incompressible fluids so unlike gaseous fuels, their mass densities do not change significantly as a function of pressure. For fluids, discharge speed V is a function of fluid supply pressure. Therefore, for equal flow areas and equal discharge speed, the mass flow rate for a gaseous fuel is much lower than that for a liquid fuel and the difference in mass flow rate becomes increasingly large as fuel supply pressure decreases. For example, at fuel supply pressures of 20 to 40 bar it is estimated that, for same injector parameters, the mass flow ratio of liquid to gas can be between 10 to 15:1.
Upon activating the actuator of a fuel injection valve, the valve member is moved to an open position that corresponds to a displacement of the actuator and allows fuel to flow through a first flow area created between the valve member and the valve seat to be injected into the combustion chamber of an internal engine, for example, at a first mass flow rate. Most conventional directly actuated fuel injection valves which are actuated by a solenoid only have one open position. For fuel injection valves actuated by a strain-type actuator more than one displacement can be commanded in which case, if a higher mass flow rate is required, the actuator can be activated to move the valve member to a second open position that corresponds to a larger flow area for injecting fuel at a higher mass flow rate. The mass ratio between the maximum and minimum fuel mass that can be injected by a fuel injection valve is called the turn down ratio. The turn down ratio is directly related to the ratio between maximum fluid flow area and minimum fluid flow area that can be achieved within an injector if the other injector, fuel, and engine parameters are kept constant. Because of the difference in mass flow rates through a given flow area between liquid fuel and gaseous fuel, it is advantageous for the injection valve employed for a bi-fuel engine operation to have a broader range of flow areas than the existing conventional fuel injection valves.
For an existing gasoline direct injection valve, for example, having a 3 mm contact diameter at the seat and employing a piezoelectric actuator that can achieve a nominal full lift of 30 to 50 microns and a partial lift of 5 to 10 microns, the ratio between the largest and the smallest flow areas achieved by the injector generally is between 3:1 and 10:1. Injection valves actuated by an electromagnetic actuator do not enable an active lift control to achieve intermediate lifts and therefore operate only between a closed and one open position.
What prevents conventional fuel injection valves from being employed as a bi-fuel injection valve is the limited range of fluid flow area. A bi-fuel injection valve needs to be capable of injecting the required amount of liquid fuel at low or idle operating conditions and the required amount of gaseous fuel at high load operating conditions. This is a very different requirement and problem than that addressed by conventional single fuel injection valves.
The “flow area” of an injector is defined herein to mean the flow area that controls the fluid flow rate during an injection event. In preferred embodiments, the flow area is the minimum cross-sectional area of the passage created when the valve member is lifted from the valve seat.
In the past, various strategies have been employed to achieve desired turn down ratios for single fuel injection valves but these strategies alone, individually or in combination, are not enough to achieve the order of magnitude difference in the range of fluid flow area that is needed for a fuel injection valve designed for switching between injecting one of either a liquid fuel or a gaseous fuel. Some of these conventional strategies include controlling the pulse width of the injection event, the fuel supply pressure or the injection valve needle lift.
The “pulse width” of an injection event is understood to be the time the fuel injection valve is open to allow fuel to be injected into the engine cylinder. Assuming a constant fuel pressure, a constant valve needle lift and a constant fuel density, a longer pulse width generally results in a larger mass of fuel being introduced into the combustion chamber. When the engine operates at idle or at low loads, the quantity of fuel required is less than what is required for other operating conditions, necessitating a shorter pulse width, which can be difficult to consistently repeat, and can then lead to variability in the amount of fuel injected. At high loads or high speeds, the pulse width can be limited by the available time for injecting the desired amount of fuel within the timing that achieves efficient fuel mixing and combustion. Accordingly, there are limits on the turn down ratio that can be achieved by only adjusting pulse width.
Adjusting the fuel supply pressure for achieving a high turn-down ratio typically results in reducing the fuel supply pressure at idle or low load and increasing the fuel supply pressure at higher engine speeds or at high loads. Reducing the liquid fuel supply pressure can be relatively easily resolved, for example, by returning a portion of the high-pressure fuel to the tank, but lowering liquid fuel supply pressure too much can inhibit the atomization of the fuel. Operating with a variable gaseous fuel supply pressure can require an additional compressor or a pump, adding to the system's complexity. For quickly reducing the gaseous fuel supply pressure, gaseous fuel can be vented from the gaseous fuel supply system, but with some gaseous fuel delivery systems it is not possible to return the gaseous fuel to the fuel storage tank, so unless the vented gaseous fuel can be captured or used by another system, some fuel might be vented into the atmosphere, which is wasteful, undesirable, and in some applications there are regulations that prohibit this.
Some types of fuel injection valves can control valve needle lift to influence the quantity of fuel that is introduced into a combustion chamber. An increase in needle lift generally corresponds to an increase in the quantity of fuel being injected. Fuel injection valves can employ a mechanical or an electrical actuator that is controllable to lift and hold the needle at intermediate positions between the closed and fully open position. Piezoelectric actuators are known in the industry to allow control of the valve needle lift at intermediate positions between the fully closed and fully open positions of the valve. With piezoelectric, magnetostrictive, and other strain-type actuators, the stroke is generally much smaller than the stroke that can be generated by electromagnetic actuators, but, on the other hand, they can generate a higher opening force and have a faster response to the activation signal, which makes the strain-type actuators more desirable for fuel injection valve applications where faster opening and closing times contribute to a better control of the fuelling. Another advantage of some strain-type actuators, for example piezo-actuators, is that they typically consume less power than electromagnetic actuators. An example of a fuel injection valve actuated by a piezoelectric actuator is described in the applicant's co-owned U.S. Pat. No. 7,527,041.
Compared to strain-type actuators, bigger displacements of valve needles can be achieved with an electromagnetic actuator, for example a solenoid. While some development has been done for fuel injection valves directly actuated by solenoids, a challenge to broad adoption has been the size and power requirements for these types of actuators for this application. Some literature that discloses the use of solenoid actuators describe fuel injection valves that use an assembly formed by two electromagnetic actuators for achieving a two-stage lift of the valve needle. For example, United Kingdom patent application number 2,341,893 describes a two-stage lift fuel injection valve for use in a common rail fuel system, that permits the lifting of the valve needle to a first intermediate position governed by the stroke of the first electromagnetic actuator, a second intermediate position governed by the stroke of the second electromagnetic actuator and a fully lifted position achieved by the combined strokes of the first and second actuators.
While current assemblies including two solenoid actuators allow holding a fuel injection valve at a few intermediate positions between the closed and open position there is still a need for more accurate and more precise control of fuel flow over a broader range of flow areas.